Filament mounting



P/ziay 12, 19361, J. M CULLOUGH FILAMENT MOUNTING Filed June. 27, 1954 INVENTORY" JACK MCCULLOUGH.

ATT RNEYS Patented May 12, 1936 UNITED STATES PATENT OFFICE FILAMENT MOUNTING Application June 27, 1934, Serial No. 732,642

3 Claims.

My invention relates to means for lessening the heat drainage from portions of heated filaments adjacent the terminals thereof.

Among the objects of my invention are; to

5 provide a means and method for the substantial elimination of heat drainage from the filaments of thermionic tubes or like devices; to provide means, for reducing heat drainage from filaments of thermionic tubes, which is applicable to tubes of existing design without materially altering said design; to provide means for increasing the power output of thermionic tubes; to provide means for procuring maximum electron emission from thermionic tube filaments; and to provide an economical construction for apparatus for the use described.

Other objects of my invention will be apparent or will be specifically pointed out in the description forming a part of this specification, but I do not limit myself to the embodiment of the invention herein described, as various forms may be adopted within the scope of the claims.

Referring to the drawing:

Figure 1 is a perspective view, partly in section, illustrating a thermionic tube cathode incorporating the prefered form of the improvements of my invention, together with other of the tube electrodes. 7

Figure 2 is a perspective view of a modified form of cathode construction also incorporating the improvements of my invention.

In broad terms, my invention is applicable to those types of electronic devices wherein a directly heated cathode is used as the source of electrons, said cathode generally comprising a filament having its ends connected to a pair of supply leads sealed through the envelope of the device, and comprises interposing means between the ends of the filament and the supply leads for substantially eliminating heat drainage from the portions of the filaments adjacent the junction thereof and the supply leads.

In practically all thermionic devices using a filamentary cathode of the directly heated type,

a distinct temperature gradient is visible when the tube is in operation, extending from the junction of the ends of the filament with the usually provided supply leads, to a point outwardly along the filament, and ranging from a barely perceptible glow adjacent the junction to full brilliancy at the above mentioned point.

This condition is caused by the relatively large diameter filament leads conducting heat from these portions of the filament faster than the filament current can build it up. In thermionic devices where this condition prevails it is impossible to obtain maximum omission from the filament at this point, and consequently the output of such a device is considerably below the maximum obtainable. It is obvious, therefore, that 5 the electron distribution, that is, the density of electrons delivered by the filament to each unit area of an electron collecting electrode will not be uniform over the surface of the electrode, hence it follows that the device is incapable of 10 operating at maximum efficiency.

I have provided means for greatly reducing, if not eliminating entirely, this objectionable heat drainage from the working portions of the filaments of devices of this character, thereby l5 obtaining greater efliciency and output than was heretofore procurable, together with better electron distribution. v

In greater detail, I have shown in Figure 1 the essential parts of a thermionic tube incor- 2O porating the preferred form of my invention. The usual anode 2 is provided surrounding the grid structure 3, these elements being disposed within the usual evacuated envelope, not shown, of which the stem 4 forms a part. The stem 25 is provided with a press 5 in which are sealed a pair of filament supply leads 6, and a central standard I. The standard is positioned to lie on the central axis of the anode and grid within the latter electrode, and'is provided adjacent its 30 upper end with a bar secured at its center to the standard, and extending on either side thereof to provide resilient arms 8 which terminate in hooks 9 adapted to engage and support the bights lfi of a pair of V shaped filaments ll. 35

Two methods of connecting the filaments to the supply leads are shown, both of which may be employed in the practice of my invention. In Figure 2 the filaments are shown as being equidistantly spaced and connected together at their 40 ends and to each of the supply leads by means of a bridge member comprising the vertical conductor i2 which is connected to the center of a horizontal conductor l3 whose ends are pinched around the ends of the filaments. 45

The method shown in Figure 1 differs somewhat from that just described in that the horizontal member l3 has been dispensed with, the ends of the filaments being brought together and connected to the supply leads by means of single link 50 members I l. This produces a symmetrical arrangement of the filaments in that they form the configuration of a tetragonal sphenoid, and also simplifies the construction by eliminating a support member, the conductor l3, with its ac- 55 companying weld, from the cathode assembly.

The filament bridge members, namely, the elements [2, I3, and M, are constructed of refractory material, preferably tantalum wire, although other metals of this nature may be employed with equally satisfactory results. It Will also be noted that the diameters of the various bridge elements closely approximate that of the filaments being of a size so that the cross section of any one element is not substantially greater than the combined cross sections of the parallel filaments and not substantially less than the cross section of one of them.

In operation, when the filaments are energized and raised to operating temperature, the temperature of the bridge elements will rise to a point where a red glow is visible therefrom, this being due to the fact that the elements are provided with current carrying capacity slightly in excess of that of the filaments. Since, therefore, the temperature difference between the filaments and the bridge elements is greatly reduced, little or no heat drainage from the former will occur, substantially all of the drainage occuring in the bridge elements themselves adjacent their junctions with the current supply leads. It is essential that the bridge elements be constructed of wire of such a size as to preclude their being raised to a temperature sufiicient to cause electron emission therefrom when the filaments are heated. This condition may be realized, for example, by constructing the filaments of thoriated tungsten, which produces satisfactory emission when operated at a temperature in the neighborhood of 1500 K. The bridge elements may be proportioned so that they will be raised to an operating temperature below this figure which is less than the electron emission temperature of tantalum, or of most of the other refractory materials suitable for use in the construction of these elements.

Tubes constructed by me, embodying my improvements showed no perceptible heat drainage from the filaments, the latter heating to working temperature over their entire lengths between junctions. It is obvious that, since this is true, tubes constructed in such a manner are capable of greater power output, and consequently more efiicient service, than tubes of similar normal capacity in present use.

The electron emission from the filament of a thermionic tube incorporating the improvements of my invention is distributed more uniformly over the surface of the collecting electrode, a condition not heretofore obtainable in devices of this character. The reason for this can be traced directly to the use of the particular bridge elements described above, which permit even heating of the filaments over their entire lengths, thereby making possible a uniform electron bombardment of each unit area of the collecting electrode. A further aid to this uniformity of electron distribution lies in the fact that the bridge elements may be located outside of the accelerating fields, thereby precluding the possibility of the obstruction of electron flow in these fields by the elements and enabling the fields to be completely supplied with electrons.

I have chosen to describe my invention in connection with thermionic vacuum tubes; however, I do not wish to be limited by this disclosure, as the improvements of my invention are readily applicable to other devices in which heated filaments are used. The main purpose of my invention is to provide a means of obtaining the maximum output from heated filaments regardless of the uses to which they may be put, such uses being readily apparent to those skilled in the art.

I claim:

1. A thermionic tube having an envelope containing a V-shaped filament heatable to produce electrons, a current supply lead sealed through said envelope and a connecting link between said lead and the apex of said V, said link being heated by the current energizing said filament to a temperature approaching that of said filament.

2. The method of energizing V-shaped electron emitting filaments which comprises the step of supplying a heating current to the apex of said V directly from a single conductor heated to a temperature approaching that of said filament.

3. The method of energizing V-shaped electron emitting filaments which comprises the steps of supplying a heating current to the apex of said V directly from a single relatively non-emitting conductor heated to a temperature approaching that of said filament.

' JACK MCCULLOUGH. 

